The apparatus and process of the invention are particularly desirable in connection with tobacco expansion processes, i.e. processes for increasing the filling capacity of tobacco. Tobacco expansion processes are used to restore tobacco bulk density and/or volume which are lost during curing and storing tobacco leaf. in addition, expanded tobacco is an important component of many low-tar and ultra low-tar cigarettes.
In current commercial processes for impregnating tobacco with an expansion agent under high pressure, for example, from 200 psig and above, the pressure vessel required is quite bulky, having heavy portable lids to withstand the pressure. The seal mechanisms for the lids are specially designed to withstand the high pressures. These types of pressure vessels, which are generally referred to as autoclaves, normally have a cylindrical body portion with convex ends, one or both ends being removable to permit loading and unloading.
One goal in any process is to increase material throughput. However, most tobacco expansion processes include a high pressure impregnation step along with other steps which cannot be carried out at high pressure. This, in turn, requires that pressure be released at some point and the treated tobacco removed from the pressure vessel. As a result, the infeed and outfeed to and from the pressure treatment step is a limiting factor in improving efficiencies in high pressure tobacco treatment processes. Thus, tobacco expansion processes employing a high pressure impregnation step are limited in their throughput efficiencies by the equipment used, particularly the pressure vessel.
Specifically, in tobacco expansion processes, a volatile tobacco expansion agent is introduced into the cellular structure of the tobacco which has collapsed due to the curing process. Generally, this step is referred to as impregnation. The impregnated tobacco is then exposed to conditions causing the expansion agent to rapidly volatilize, causing the tobacco cell to expand as the compound is driven out of the cell in a gaseous or vaporous state. Volatilization of the expansion agent is accomplished by heating the impregnated tobacco in many cases or by rapidly reducing pressure in other cases. There are a number of processes which utilize these basic concepts with different expansion agents, some of which are disclosed in U.S. Pat. No. Re. 30,693, U.S. Pat Nos. 3,524,452; 3,771,533; and 4,531,529; British Patent Specification No. 1,484,536 and Canadian Patent No. 1,013,640.
The amount of pressure used to impregnate the tobacco generally depends on the particular expansion agent employed. U.S. Pat. No. 3,524,452 to Stewart et al. discloses a process in which a relatively low pressure can be used because the impregnant is normally in a condensed state at these pressures, while Canadian patent No. 1,013,640 and British Patent Specification No. 1,484,536, which disclose processes which use carbon dioxide as the impregnating compound, and require a much higher pressure to ensure that carbon dioxide is introduced into the tobacco cells in sufficient quantity to cause expansion of the cells when the impregnated tobacco is heated.
Some of the drawbacks of using any of these and other prior art high pressure systems are the bulkiness of the autoclave and lids, the difficulties with sealing the system, the special basket or container required to hold tobacco, and other problems associated with loading and unloading tobacco into and out of the pressure vessel.
U.S. Pat. No. 4,554,932 to Conrad and White, incorporated herein by reference, describes a fluid pressure treating apparatus including a tubular shell housing a spool assembly. The spool includes a cylindrical spool body portion that is preferably of relatively small diameter that extends between the two spool ends. The spool ends have a diameter greater than the spool body, but less than the inside diameter of the tubular shell. The spool is mounted for reciprocating movement between a loading position outside the shell, a treating position within the shell, and an unloading position outside of the shell. When the spool is within the shell, deformable sealing rings carried in annular grooves on the spool ends are forced radially outwardly for engagement with the interior of the shell wall. This provides a sealed, annular-shaped pressure chamber inside the shell, in the space between the spool ends and surrounding the smaller spool body. One or more ports through the shell cooperate with conduit shaped cavities extending radially into one or both spool ends and axially along the spool body, to allow input and removal of processing fluids into and from the annular space around the spool body within the shell.
U.S. patent application Ser. No. 08/163,049 filed Dec. 6, 1993, to Beard et al., entitled Tobacco Expansion Process and Apparatus, now U.S. Pat. No. 5,469,872 describes an apparatus and process for expanding tobacco at rapid throughput rates employing high pressure tobacco impregnation conditions. A preferred apparatus according to that invention employs the concepts of the pressure vessel including the spool and shell assembly of U.S. Pat. No. 4,554,932 set forth above. An improved spool assembly disclosed therein includes elastomeric sealing rings attached in annular grooves about the periphery of the end members of the spool, as well as wear rings to narrow the annular space or gap between the spool assembly and the shell. These sealing rings are integral with the wear rings and are exposed to a high pressure fluid, typically a food grade vegetable oil, on their inside circumferential surface to cause the rings to expand radially outwardly to accomplish their sealing function.
Although the spool and shell pressure vessel produces substantial time savings and improve economics in tobacco expansion, the fluid used to expand the sealing rings must be ported to the sealing rings by providing blind ports within the spool body. Moreover, the rings must be periodically replaced by removing the old rings and bonding new rings to the spool body. If the elastomeric ring pressure fluid such as vegetable oil, leaks onto the tobacco, usefulness of the tobacco in the manufacture of cigarettes is impaired. Moreover, replacement of worn sealing rings is time consuming and costly.